Consider the following acid-base reaction: (Fe^3+)(aq) + (3H20) <=> (Fe(OH)3 (s)

Question

Consider the following acid-base reaction:

(Fe^3+)(aq) + (3H20) <=> (Fe(OH)3 (s)) + (3H+)

a) For thermodynamic considerations calculate the equilibrium constant K at 25 degrees C

b) For the value of K you calculated in part a, if a solution contains 10^-4 M Fe^3+ and has a pH of 7.5, will Fe(OH)3 (s) form? Show all calculations.

3.14 Consider the following acid-base reaction: Fe(a3O Fe(OH),()3H From thermodynamic considerations calculate the equilibrium constant K at 25. (a) (b) For the value of K you calculated in part (a), if a solution contains 10- M Fe and has a pH of 7.5, will Fe(OH)() form? Show all calculations necessary to justify your answer.

Explanation / Answer

1) Given reaction:

Fe3+ + 3H2O = Fe(OH)3(s) + 3H+

From standard data, for this reaction, logK = 3.191

K =10-3.191 = 0.000644 = 6.44*10-4

2) Given pH = 7.5, so pOH = 14-7.5 = 6.5 and OH- = 10-6.5 = 3.16*10-7 M

Fe3+ = 10-4M

Fe(OH)3(s) Fe3+(aq) + 3OH- (aq)

Now, Ksp = [Fe3+] [OH]3 = (10-4)(3.16*10-7)3

Ksp = 3.16*10-24

Since, Ksp < K

  3.16*10-24 < 6.44*10-4

Since this value for K is greater than the Ksp, you would expect precipitation to occur

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